Insights into air pollution chemistry and sulphate formation from nitrous acid (HONO) measurements during haze events in Beijing - RSC Publishing
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Faraday Discussions
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Insights into air pollution chemistry and
Open Access Article. Published on 21 September 2020. Downloaded on 09/01/2021 16:43:50.
sulphate formation from nitrous acid
(HONO) measurements during haze events
in Beijing†
William J. Bloss, *a Louisa Kramer,‡a Leigh R. Crilley,§a Tuan Vu,{a
Roy M. Harrison, a Zongbo Shi, a James D. Lee, b
Freya A. Squires, b Lisa K. Whalley,c Eloise Slater,c Robert Woodward-
Massey,kc Chunxiang Ye,kc Dwayne E. Heard, c Shengrui Tong,d
Siqi Hou,d Yele Sun, e Jingsha Xu,a Lianfang Weie and Pingqing Fu f
Received 13th August 2020, Accepted 21st September 2020
DOI: 10.1039/d0fd00100g
Wintertime urban air pollution in many global megacities is characterised by episodic rapid
increase in particulate matter concentrations associated with elevated relative humidity –
so-called haze episodes, which have become characteristic of cities such as Beijing.
Atmospheric chemistry within haze combines gas- and condensed-phase chemical
processes, leading to the growth in secondary species such as sulphate aerosols. Here,
we integrate observations of reactive gas phase species (HONO, OH, NOx) and time-
resolved aerosol composition, to explore observational constraints on the mechanisms
responsible for sulphate growth during the onset of haze events. We show that HONO
abundance is dominated by established fast gas-phase photochemistry, but the
consideration of the additional formation potentially associated with condensed-phase
oxidation of S species by aqueous NO2 leading to NO2 production and hence HONO
release, improves agreement between observed and calculated gas-phase HONO
a
School of Geography, Earth and Environmental Sciences, University of Birmingham, UK. E-mail: w.j.bloss@
bham.ac.uk
b
Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, UK
c
School of Chemistry, University of Leeds, UK
d
Institute of Chemistry, Chinese Academy of Sciences, Beijing, China
e
State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of
Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
f
Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0fd00100g
‡ Currently at Ricardo Energy & Environment, Harwell, Oxfordshire, OX11 0QR, UK.
§ Currently at the Department of Chemistry, York University, Toronto, ON, Canada.
{ Currently at the School of Public Health, Imperial College London, UK.
k Currently at the College of Environmental Sciences and Engineering, Peking University, Beijing,
100871, China.
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levels. This conclusion is highly dependent upon aerosol pH, ionic strength and particularly
the parameterisation employed for S(IV) oxidation kinetics, for which an upper limit is
derived.
Background
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Atmospheric chemical processing leads to the removal of primary pollutants and
determines the rate of production of secondary species, including ozone and
many components of aerosol particles. Gas-phase processing is driven by a small
number of key oxidants, of which the OH radical is predominant for the majority
of species during daytime, including many hydrocarbons and inorganic gas-phase
species such as SO2 and NO2. Gas- and condensed-phase oxidation chemistry are
coupled across the two phases of aerosol, with a wide range of aqueous oxidants
driving the particle phase oxidation of species such as SO2, including NO2 itself.
In many urban areas, particulate matter (PM, particularly PM2.5) is a key
regulated air pollutant, whose abundance drives substantial adverse human
health impacts, in addition to affecting visibility, radiation transfer and amenity.
PM is formed of a complex chemical mixture including both directly emitted
(primary) and chemically produced (secondary) components; understanding the
relationship between these underpins the accurate identication of direct and
indirect PM sources, formulating the most effective and efficient clean air policy.
This challenge is emphasised by the air pollution climatology of Beijing and
the North China Plain, where seasonal haze events characterised by episodic
PM2.5 excursions to 500 mg m3 and above, impact the health and wellbeing of
around 400 million people. Sulphate is known to be a major component of the
ne size fraction of Beijing haze. The rate of formation of sulphate aerosol from
SO2, driven by chemical oxidation, is observed to increase as PM2.5 levels rise, and
recent studies into haze formation have found that the rate of production of
sulphate aerosol is signicantly accelerated in megacities such as Beijing.1 New
chemical mechanisms have been proposed, notably including the aqueous phase
oxidation of SO2 by NO2, if aerosol pH is sufficiently high. A by-product of this
mechanism is expected to be the production of gas-phase HONO.
Here, we analyse in situ observations collected during the UK–Chinese Air
Pollution and Human Health (APHH) campaign in central Beijing during haze
episodes in November–December 2016, which constrain the gas-phase produc-
tion and removal of HONO, and the observed rate of formation of sulphate. We
identify constraints from the in situ observations upon potential chemical SO2
oxidation mechanisms contributing to haze formation.
Sulphate aerosol formation in haze
Secondary SO4 formation is driven by a number of chemical mechanisms.
Globally, the dominant process is the gas-phase reaction of SO2 with OH radicals,
leading to the formation of HSO3 intermediates, which rapidly react with O2
forming SO3, which is converted to H2SO4 through reaction with water vapour.
The recently characterized rapid reaction between SO2 and criegee intermediates
formed through alkene ozonolysis,2 is unlikely to signicantly impact boundary
layer SO2 oxidation due to the rapid removal of criegee intermediates by water
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vapour and the water vapour dimer.3,4 SO2 may also partition to aqueous aerosol
particles, where a series of equilibria partition between SO2$H2O (at low pH),
HSO3 (pH 2–7) or SO32 under basic conditions. In the aqueous phase, SO2
processing is driven by H2O2 (when pH is near neutral) and O3 (of increasing
importance under more acidic conditions). Transition metals (Fe, Mn) can also
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catalyse O2 reactions leading to condensed phase sulphate formation. During the
onset of haze events, sulphate formation rates of the order of tens of mg per m3 per
Open Access Article. Published on 21 September 2020. Downloaded on 09/01/2021 16:43:50.
hour are observed.1 Very recently, Wang et al.5 reported the potential for HONO-
mediated SO2-to-SO4 oxidation in Beijing cloud/fog, with associated N2O
formation.
However, current air quality models fail to reproduce elevated sulphate levels
in winter haze. Several mechanisms for S(IV) oxidation are still under consider-
ation to account for the underestimation of sulphate concentration between
model simulations and eld observations, including reactive nitrogen chemistry
involving the oxidation of S(IV) by NO2,6–10 underestimation of the transient metals
catalyzed pathway,11,12 the possible oxidation on the surface of acid droplets by
O2,13 or the otherwise dominant role of S(IV) + H2O2 pathway.14 Although these
pathways can improve the agreement between sulphate simulation and
measurement, there remains a lack of independent constraints on modelling
results, especially the potentially dominant role of the NO2–S reaction. A tagging
methodology with triple oxygen isotopes has been applied as a proxy to constrain
the relative contributions of the individual oxidation pathways of S(IV),12,15 with
results showing that the NO2–S reaction is highly dependent on pH and dominant
under high-pH conditions, however, a very substantial uncertainty resulted from
the assumed activity coefficient between the aerosol liquid phase ionic strength
and parameterisation of the kinetic reaction rate.
The reaction between NO2, partitioned into the aqueous phase, with HSO3
remains a potentially dominant driver of sulphate formation under haze condi-
tions, forming SO42 at rates of 1–7 mg m3 h1:6,8
2NO2(aq) + HSO3(aq) + H2O(aq) / 3H(aq)+ + 2NO2(aq) + SO4(aq)2 (1)
This is in contrast to tropospheric cloud processing of sulphur species, which
is dominated by the H2O2 and O3 reactions, with NO2 processing relatively
unimportant.16 The mechanism has the potential to drive a positive feedback in
haze formation, where increased sulphate production leads to increased aerosol
water and hence SO2 uptake and oxidation, under conditions of high NO2.6 Of
particular note here, the condensed phase nitrite is expected to be released from
acidic aerosol to the atmosphere as HONO.8
The ionic composition of the aqueous aerosol is critically important in
calculating the importance of the NO2 mechanism – ionic strength (which
impacts the rate constants for the S(VI) oxidation reactions) and aerosol pH.
Elevated pH enhances SO2 solubility (higher effective Henry’s law constant);
consequently chemical factors to maintain elevated aerosol pH (5–7) are required,
and have been attributed to high levels of ammonia emissions/ammonium in the
North China Plain,6,8 However, subsequent analyses have concluded that the
highest observed NH3 levels are insufficient to raise pH levels above 4.5 under
haze conditions, and hence it is unlikely that NO2-mediated oxidation of SO2 is
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a major sulphate formation route in Beijing haze, compared with O3 and
transition-metal mediated oxidation.11
Boundary layer HONO photochemistry
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The basic gas-phase chemistry of HONO combines formation through the
homogenous termolecular OH + NO reaction (R2), and reaction of HONO with OH
Open Access Article. Published on 21 September 2020. Downloaded on 09/01/2021 16:43:50.
(R3) and relatively rapid solar photolysis (R4), where the photolysis frequency (1.7
103 s1 at SZA ¼ 40 ) corresponds to a typical mid-latitude local solar noon
lifetime of around 10 min.
OH + NO + M / HONO + M (2)
HONO + OH / H2O + NO2 (3)
HONO + hn / OH + NO (4)
HONO photolysis (R4) is an important, and frequently the dominant, source of
OH radicals in the continental boundary layer, accounting for 40% of OH
formation in central London,17 over 80% of OH production at a rural site in
Colorado,18 and most recently an average of 83% of OH production in wintertime
Beijing19 – hence, HONO abundance and formation mechanisms are tightly
coupled to the production of OH and rate of in situ gas-phase oxidation for many
atmospheric pollutants.
HONO levels may be predicted by reactions (R2)–(R4), assuming a photo-
chemical steady state (see below), which may then be compared with observed
concentrations to ascertain the presence of additional HONO sources. This
exercise has been undertaken a number of times, leading to a range of additional
HONO sources being identied – which are frequently signicantly larger than
the homogeneous OH + NO reaction. Examples include source strengths
exceeding reaction (R2) by factor 13 in a European forest,20 30 at Hyytiälä21 and
a factor of 19 in rural China.22 In performing such analyses, it is important to note
the timescales for HONO are rather longer than those of OH, hence analysis in
highly spatially heterogeneous environments can be challenging as noted by Lee
et al.23 and Crilley et al.24
In general terms, such sources may be combined as an additional source term,
SHONO, if HONO may be placed into the steady state:
SHONO / HONO (5)
d[HONO]/dt ¼ k2[OH][NO] + SHONO k3[OH][HONO] j4[HONO] (E1)
A number of mechanisms for further atmospheric HONO emission and
production have been suggested. Direct emission sources include vehicle
exhausts (with HONO emissions equivalent to 1.24%, and 0.85% of NOx emis-
sions recently derived for the Hong Kong and UK vehicle eets, respectively),25,26
biocrusts,27 and microbial soil emissions.28 Heterogeneous HONO sources
include dark reactions between NO2 and water (e.g. Finlayson-Pitts et al.29) on
ground and aerosol surfaces, photolysis of nitrophenols, and photoenhanced
conversion of NO2 to HONO in the presence of organic chromophores.30
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Substantial work has been undertaken and the reader is directed to the reviews of
Kleffman31 and Spataro and Ianniello.32
The UK–Chinese Air Pollution and Human Health – Beijing experiment
(APHH-Beijing), was funded by the UK Natural Environment Research Council
and the National Science Foundation of China. APHH-Beijing brought together
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well over 100 instruments to study boundary layer atmospheric composition and
haze formation at the Institute of Atmospheric Physics (IAP), Chinese Academy of
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Sciences in central Beijing in November–December 2016 and spring–summer
2017.
Here, we combine co-located measurements from the APHH study, alongside
numerical model calculations, to explore potential constraints on the sulphate
formation mechanisms under haze conditions in Beijing. We compare the
abundance of HONO with potentially determinant parameters including PM,
meteorological factors and aerosol composition (including SO4). We calculate the
HONO concentrations which would be predicted in ambient air from established
gas-phase chemistry (reactions (2)–(4)), and the rate of HONO formation from the
aerosol-mediated NO2/SO2 oxidation process (reaction (1)). We compare predicted
HONO levels with those observed, and discuss the quantitative agreement in the
context of different approaches to parameterising the NO2-enhanced SO2 oxida-
tion kinetics. Implications for air quality are discussed.
APHH-Beijing measurements
Measurements were acquired during the APHH-Beijing winter experiment, con-
ducted from 10 November–10 December 2016 at the Institute of Atmospheric
Physics (IAP), Chinese Academy of Sciences in central Beijing 39 580 3300 N,
116 220 4100 E, between the third and fourth North ring roads. The site is in
a residential area, with a number of substantial highways ca. 150 m away to the
south, north and west. On the site itself, an open grassed area is surrounded by 2–
3 storey buildings, with larger structures (residential blocks) some 200–300 m
away. A canal runs past the north perimeter of the site, with a park (including
conifer pine trees) further to the west. Given recent work to relocate industry from
central Beijing, the dominant local emission sources are anticipated to be traffic
and domestic activities, with possible contributions from light industry, for NOx.
For SO2, sources are anticipated to be regional, industry and power/heating-
related combustion (including coal) around the city periphery. For full details
of the site, instruments and campaign overview, including a detailed meteoro-
logical timeseries, see Shi et al.33
OH radicals were measured via laser-induced uorescence, using the uores-
cence assay by gas expansion (FAGE) methodology. Full details of the instrument
operation and calibration are given in Slater et al.19 NO was measured using
chemiluminescence employing a Thermo TE42i NOx monitor, calibrated to an
NPL a-standard. Photolysis frequencies (jHONO) were measured at ground level
using a calibrated spectral radiometer. HONO was measured using the LOPAP
(Long Path Optical Absorption Photometry) approach. The LOPAP is a wet
chemical technique and has been described in detail in Heland et al.34 and
Kleffmann et al.35 Briey, a stripping coil entrains gas-phase HONO into an acidic
solution where it is derivatized into an azo dye, detected by long path absorption
at 550 nm, and calibrated against an aqueous phase nitrite standard. The LOPAP
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was operated and calibrated according to the standard procedures described in
Kleffmann and Wiesen.36 A detailed account of the APHH-Beijing HONO dataset,
including an intercomparison of HONO measurements by LOPAP, broadband
cavity absorption spectrometry, chemical ionisation mass spectrometry and
proton transfer reaction mass spectrometry is given in Crilley et al.37 All data were
acquired at ground level (inlet heights ¼ 5 m).
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Aerosol data for sulphate mass concentration were taken from AMS (aerosol
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mass spectrometer) measurements; these therefore correspond to the ne frac-
tion (PM1) of the particulate matter present. The sulphate concentrations deter-
mined by AMS data were compared with the results obtained by 24 hour high-
volume PM2.5 lter samples collected at the IAP site during December 2016,
and analysed in the laboratory by ion chromatography.38 The two datasets were
found to be within 20% of each other (r2 ¼ 0.97) and the AMS data were used
without further adjustment. PM2.5 mass concentrations were derived by TEOM-
FDMS, and were recorded at the top of a two-storey building (about 10 m above
ground level). Data were averaged to hourly mean mixing ratios using in situ
temperature and pressure measurements. The period 2–10 December was
selected as this encompassed several periods of haze, with rapid and dramatic
PM2.5 growth reaching nearly 500 mg m3 (haze periods typically identied as
PM2.5 > 75 mg m3), and maximised overlap between the operational periods of
the various instruments used – during this date range, 65% of the necessary
Fig. 1 Measured timeseries for HONO (ppb), NO (ppb), NO2 (ppb), OH (molec cm3), SO2
(ppb), jHONO (s1), SO4 (AMS; mg m3) and PM2.5 (TEOM FDMS; mg m3) at the IAP tower site
during the APHH Beijing winter campaign, 2016.
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measurements were present (see Fig. 1), limited primarily by the OH and HONO
datasets. Conditions during this period were typical of the wider APHH winter
campaign (see Shi et al.33 for climatological details).
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Analysis of in situ observations
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Fig. 1 shows the characteristic pattern of haze event onset observed previously,
and associated with increases in many air pollutant species and intermediates;
daily mean mixing ratios/concentrations are given in ESI Tables ST1 and ST2.†
There is a positive correlation between HONO and sulphate abundance, indi-
cating commonality in their mechanisms of formation/removal. In general,
HONO abundance is frequently closely coupled with that of NO2 (through
heterogeneous NO2-to-HONO conversion chemistry, and/or co-emission of both
species), and variations in the HONO/NO2 ratio can be used to characterise this
relationship. We nd HONO levels are correlated with the abundance of PM2.5
(r2 ¼ 0.67) and of aerosol sulphate (r2 ¼ 0.59), but the HONO/NO2 ratio is much
less strongly related to PM2.5 abundance (r2 ¼ 0.27), indicating that the interac-
tion between particulate matter and HONO may be more strongly related to
sulphate chemistry, than NO2-to-HONO heterogeneous conversion. The HONO–
SO4 relationship does not show any dependence upon SO2, but higher levels of
both species are strongly associated with higher RH (Fig. 2), indicating a potential
association with aqueous content, such as the condensed phase SO2 oxidation
mechanisms outlined above.
The measurement data coverage during the APHH-Beijing campaign at IAP
provides direct constraints on the major (gas-phase, photochemical) processes
forming and removing ambient nitrous acid, including measurements of
concentrations of OH, NO, HONO and meteorological/photolysis parameters. We
averaged data to an hourly time resolution, and evaluated the consecutive
changes in concentration of HONO (i.e. d[HONO]/dt, dened simply as the
difference in consecutive hourly mean measurements of HONO abundance), and
Fig. 2 Relationship between HONO and SO4 concentrations, coloured by SO2 abundance
(panel A) and by relative humidity (RH, panel B), during the 2–10 December focus period.
The data show the limited impact of SO2, but the highest levels of both HONO and SO4 are
associated with elevated humidity.
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rate of the chemical reactions forming and removing HONO over the equivalent
time periods (E1).
Fig. 3 shows the derived timeseries for d[HONO]/dt, and the individual
components of HONO formation and removal, focusing upon the case study
period of 2–10 December. The dominance of the conventional gas-phase chem-
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istry is apparent, with HONO formation dominated by the OH + NO reaction, and
removal by photolysis, with a small to negligible contribution from the OH +
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HONO reaction.
Discounting potential additional sources of HONO (i.e., assuming SHONO ¼ 0),
we can calculate the HONO abundance that would be predicted assuming the
steady state applies. We limit the analysis to daylight only (here, hourly-averaged
data where the HONO lifetime is less than 60 min), corresponding to jHONO > 2.78
104 s1, and where all measurements are available (HONO, OH, NO, SO2,
photolysis parameters, aerosol composition) – this condition reduces the avail-
able data to 44 hourly averages, 23% of the total over the focus period (the
principal reduction vs. the coverage noted above arises due to the restriction to
daylight hours). The predicted HONO concentration is compared with that
observed in Fig. 4. The PSS-calculated (red triangles) and observed (blue circles)
HONO mixing ratios are in relatively good agreement, indicating limited imme-
diate need for additional production terms. A clearer picture is obtained from the
regression analysis (Fig. 5), comparing the PSS-calculated and observed HONO
mixing ratios (red triangles). While the data are scattered, overall the observed
and calculated mixing ratios are similar in overall magnitude, without evidence
for orders-of-magnitude error. Regression analysis derives gradient and intercept
values of 0.90 0.38 and (0.59 0.84) ppb respectively (2s), with an r2 coeffi-
cient of 0.35. HONO measurements performed by multiple independent tech-
niques during the APHH-Beijing study have been compared by Crilley et al.,37 who
showed that the LOPAP-derived HONO data had a detection limit of around 35
ppt (i.e. very much smaller than the scatter of the observations), and that all
instruments (using different techniques) were extremely well correlated with each
other (r > 0.97), but with variations in slope (i.e. absolute accuracy) of 0.61–0.88.
Fig. 3 Timeseries of the observationally-derived components of in situ HONO formation
and destruction: reactions of OH + NO (R2), OH + HONO (R3), HONO + hn (R4), plus the
observed rate of change of HONO, d[HONO]/dt.
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Fig. 4 Timeseries of observed daytime HONO mixing ratios (blue circles), steady state
calculated HONO mixing ratios – HONO PSS, as derived from application of eqn (E1) –
(red triangles) and model calculated HONO (see text; green squares).
Fig. 5 Correlation between observed HONO and PSS-predicted mixing ratios (red
triangles), regression line (red dashes), and model calculated HONO levels (see text; green
squares) and associated regression (green dots).
Therefore the scatter in the observed data in Fig. 4 does not reect measurement
precision (although the observations may collectively be biased high or low,
depending on the absolute accuracy), and rather reects changes in the actual
observed concentrations, in turn reecting variations in the atmospheric source
and sink-terms as integrated over the HONO lifetime.
The data can also be analysed to quantify HONO formation (or removal)
mechanisms not accounted for by the basic chemistry (reactions (R2)–(R4))
through application of (E1) alongside the observed d[HONO]/dt to derive SHONO,
however the inferred net source for HONO, SHONO, is very scattered and displays
no correlation with SO2 or SO4 abundance (r2 ¼ 0.04 and 0.02 respectively), or with
the SO2-normalised rate of SO4 formation (r2 ¼ 0.05) – not shown. A crude esti-
mation of the rate of formation of HONO (technically, NO2) which would be
associated with SO2 oxidation (Psulphate) can be obtained from the observed rate of
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formation of sulphate, d[SO4]/dt, multiplied by a factor of 2 for the stoichiometry
of NO2 formation:
Psulphate ¼ 2 d[SO4]/dt (E2)
Critically, in addition to disregarding other (non–NO2–mediated) mechanisms
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for SO2 oxidation, this estimate for HONO formation associated with SO4
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production neglects the effect of dynamics, i.e. advection of more (or less)
polluted airmasses across the study site – leading to negative as well as positive
values (Fig. S1†). Nonetheless it is instructive to compare the resulting daytime
Psulphate values, which ranged from 0.2 to 2.84 ppb h1, to those predicted for the
OH + NO reaction (Fig. 3) – the Psulphate values are nearly one order of magnitude
smaller (and likely signicant overestimates during periods of haze growth, due
to pollution advection).
Model quantification of SO2 oxidation pathways
To more comprehensively quantify the relative importance of different SO2
oxidation mechanisms and the interaction with HONO formation, a zero-
dimensional numerical model constrained to the observed conditions was used
to calculate the gas- and condensed-phase SO2 oxidation rates for the focus
period. The model framework involved a series of processes, consisting of gas-
phase reaction of SO2 with OH, absorption equilibria of gaseous species, ioni-
zation, aqueous phase chemical reactions of S(IV), and the kinetics of mass
transport. The model assumed that air infuses into the aerosol water and then
participates in chemical reactions. The condensed-phase oxidation of S(IV) by
H2O2, O3, oxygen catalyzed by Fe3+/Mn2+ or NO2 in the aerosol water were
explicitly represented. The inuence of primary sulphate emission and regional
transportation were disregarded – the focus is on in situ chemical production
only. The kinetics of sulphate formation and its dependence on ionic strength of
aerosol water were implemented following Cheng et al.6 and He et al.15 Concen-
trations of Fe3+ and Mn2+ were pH-dependent and followed the precipitation
equilibria of Fe(OH)3 and Mn(OH)2.
The Nested Air Quality Prediction Model System (NAQPMS) developed by the
Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP/CAS) was
adopted to simulate hourly concentrations of OH free radicals and H2O2 during
the eld campaign.39 NAQPMS incorporated the Carbon Bond Mechanism version
Z (CBM-Z)40 to simulate gas-phase reactions. The modeled OH and H2O2
concentrations were used in the estimation of sulphate formation from the gas-
phase reaction of OH with SO2 and the aqueous-phase reaction of H2O2 with
HSO3, and (in the case of OH) to evaluate the model oxidant eld calculation.
The observed aerosol composition, gaseous NH3 and meteorological conditions
were input into the ISORROPIA-II thermodynamic equilibrium model41 to esti-
mate the aerosol water content (AWC), aerosol water pH, and ionic strength
(Table ST2†). The model was run in forward mode and predicted the phase par-
titioning of a K+/Ca2+/Mg2+/NH4+/Na+/SO42/NO3/Cl/H2O in thermodynamic
equilibrium with gas-phase precursors.
By way of validation, the modelled and observed OH levels19 as a sensitive test
of the integrated NAQPMS oxidation (VOC, NOx and O3 chemistry), are compared
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in Fig. S2.† Overall, agreement was good, with the model showing good skill in
reproducing the diurnal behaviour of OH. The absolute modelled OH levels were
within 15% of those observed on 4 of the 6 days considered, but exceeded those
observed (by up to a factor of 2) on the remaining 2 days (interestingly, those
characterised by the lowest NO levels).
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The modelled rate of formation of SO4 was determined for OH (gas phase),
H2O2, O3, transition metals (Fe, Mn catalysed O2 reaction) and NO2 driven
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processes. Ionic strength is a key factor in such calculations: sulphate production
rates were calculated for three cases: (A) neglecting the impact of ionic strength;
(B) considering the impact of ionic strength upon the solubility of SO2 and the
dissociation of H2SO3 (ref. 42) and upon the kinetics of the H2O2, O3 and tran-
sition metal reactions; and (C) also considering the impact of ionic strength upon
the rate constant for the NO2 + S(IV) reaction, following the parameterisations of
Cheng et al.6 We compared these approaches as there is a lack of experimental
data for the inuence of ionic strength upon the NO2–S reaction, and the calcu-
lated aerosol ionic strength during severe haze events reaches 60 M (when RH >
40% and PM2.5 > 75 mg m3) hence extrapolations from the conditions of dilute
solutions may not be warranted.
Discounting ionic strength effects (i.e., case A), SO4 formation averaged over
the whole 12 November–10 December 2016 campaign period is found to be
dominated by gas-phase OH chemistry (51%), followed by condensed phase H2O2
and transition metal reactions, with NO2 reaction responsible for 1.6%. Consid-
ering ionic strength effects other than NO2 kinetics (case B), oxidation of SO2 (g)
by OH drives 81% of SO4 formation, with 18% arising from H2O2 and 0.6% for
reaction with NO2 – however the average and maximum rates of SO4 formation
calculated were 0.036 and 0.29 mg m3 h1 respectively, much lower than those
observed. Inclusion of the ionic strength parameterisation for the NO2 + S kinetics
(case C) increases the calculated rate of SO4 formation by two orders of magnitude
(a factor of 96), with the calculated contribution of the NO2 pathway for SO4
Fig. 6 Modelled SO4 formation rates (converted to ppb h1) through reaction of SO2 with
OH (orange line), condensed phase reactions (all case B) with H2O2 (grey), O3 (yellow),
transition metals (blue) and NO2 (green). Dashed line, second y axis: NO2 reaction
considering impacts of ionic strength on NO2 + S(IV) reaction kinetics (i.e. case C). See
Fig. S5 (ESI†) for expansion of the H2O2, O3 and TMI timeseries.
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Faraday Discussions Paper
formation increasing to 98% overall, as a consequence of the very strong
dependence or the rate constant upon ionic strength, highlighting this critical
uncertainty in this aspect of the kinetics parameterisation. The average and
maximum rates of SO4 formation calculated for case C were 3.5 and 12 mg m3
h1, respectively, comparable to the behaviour observed (Fig. 1 and S3†).
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The timeseries for the modelled SO4 formation rates for each chemical
mechanism under case (B), and for the NO2-driven processing under case (C), are
Open Access Article. Published on 21 September 2020. Downloaded on 09/01/2021 16:43:50.
shown in Fig. 6 for the 2–7 December focus period – note the different y axis scale
used for the latter data series (see also Fig. S4†). The signicant increase in the
overall rate of formation of SO4 for case (C) is in much closer accord with the rates
of SO4 formation derived from the observed timeseries (Fig. 1), than for cases (A)
and (B).
Considering the rate of production of NO2, as an estimate to the HONO
formation rate associated with NO2/SO2 oxidation, it is clear that the non-ionic-
strength enhanced kinetics (case (A) and (B) – the latter as show in Fig. 6) are
too small to make any appreciable impact upon HONO production, compared
with the dominant OH + NO reaction ux. Calculated HONO formation from case
(C), dominated by the ionic-strength-dependent reaction with NO2, was added to
the homogeneous OH + NO production to determine the impact upon calculated
steady state HONO concentrations, according to eqn (E3), and as shown in Fig. 4
and 5 as HONO (+Model).
[HONO] ¼ {k1[OH][NO] + [Modelled SO2 + NO2 rate]}/{k2[OH][HONO]
+ j3[HONO]} (E3)
Addition of the SO2 + NO2 derived HONO ux increases the HONO levels by
27% on average, and modestly improves agreement between the calculated and
observed HONO levels (r2 ¼ 0.98). The observed and PSS-calculated HONO levels
are compared as a function of SO4 in Fig. S3† – while higher levels of both are
associated with more polluted conditions (more PM2.5, more SO4), no bias
between the observed and PSS-calculated levels with respect to SO4 is apparent,
indicating this factor is not dominant. By applying a scale factor to the model-
derived HONO production component, it can be shown that the rate of SO2
oxidation by NO2, as simulated here, is consistent with the observed HONO
within the scatter of the data, but would cease to be so if the ionic strength
enhancement factor were to be signicantly increased – an upper limit (corre-
sponding to a regression gradient of 1.5) of a factor of 4.5 fold higher may be
estimated.
Conclusions
Nitrous acid was measured in the urban atmosphere of Beijing, at mixing ratios of
up to 10 ppb (signicantly higher than many other cities, e.g. 1.6 ppb for Lon-
don17). HONO abundance, alongside that of many air pollutant species, was
correlated with the formation of episodic haze events, associated with rapid
growth in PM2.5 under elevated RH conditions, including increases in sulphate
concentration. The HONO abundance was correlated with that of SO4, and with
RH. The HONO/NO2 ratio, a marker for heterogeneous interconversion of reactive
nitrogen oxides to HONO through dark- and photo-enhanced reaction processes,
234 | Faraday Discuss., 2021, 226, 223–238 This journal is © The Royal Society of Chemistry 2021View Article Online
Paper Faraday Discussions
was not correlated with aerosol abundance, indicating heterogeneous formation
does not dominate HONO production. Rather, during daytime, fast conventional
gas-phase chemistry – the OH + NO reaction (2), and HONO photolysis (4) –
dominates the HONO abundance in wintertime Beijing, and considering these
processes (and the minor HONO + OH reaction (R3)), permits alone good quan-
titative agreement (regression coefficient ¼ 0.90 0.38) between measured and
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calculated photostationary steady state HONO levels.
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Combination of observationally-constrained rates of HONO formation and
loss through reactions (2)–(4), with the calculated rates of HONO production
through heterogeneous mechanisms including H2O2, O3, transition metal and S–
NO2 chemistry showed that, discounting the effects of ionic strength, the
condensed phase mechanisms made a negligible contribution to HONO forma-
tion, while SO2 oxidation was dominated by gas-phase OH chemistry, with the S–
NO2 reaction contributing 1.6% of the total. Inclusion of the ionic strength
effects, including parameterisation of the NO2–S(IV) kinetics following Cheng
et al.,6 led to calculated condensed-phase nitrite production equivalent to
increases in the steady state HONO levels of 27%, modestly increasing agreement
with the HONO levels observed. Under this scenario, the NO2–S processing
dominates SO4 formation. Considering the constraint the HONO levels place
upon the additional production attributable to the NO2–S reaction, an upper limit
to the maximum possible enhancement of the ionic-strength-dependent kinetics
of a factor of 4.5 is estimated.
This work demonstrates the potential, and limitations, for coordinated
observations of gaseous and particulate atmospheric composition to constrain
fundamental processes which are extremely challenging to evaluate accurately in
the laboratory or eld – in this case, aerosol liquid water pH and ionic strength
kinetic behaviour under very strongly non-ideal conditions. A key limitation of the
data which are available here is their xed measurement location: a lagrangian
approach, allowing parameters to be evaluated along the trajectory of an evolving
airmass at higher altitude and remote from possible surface inuence may
facilitate greater insight.
Conflicts of interest
There are no conicts of interest.
Acknowledgements
This work was funded by the UK Natural Environment Research Council (NERC),
the Medical Research Council and Natural Science Foundation of China under
the framework of the Newton Innovation Fund (NE/N007190/1 and NE/N007077/
1), and by the UK NERC through the project Sources of Nitrous Acid in the
Atmospheric Boundary Layer (SNAABL, NE/M013405/1). We acknowledge the
support from Zifa Wang and Jie Li for hosting the APHH-Beijing campaign at IAP,
and thank the whole APHH team for their support throughout the project
campaigns in Beijing and beyond.
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Faraday Discussions Paper
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